1. Field of the Invention
The present invention relates to an optical wavelength multiplexer/demultiplexer used in a wavelength-division multiplexing (WDM) system, and more particularly to an optical wavelength demultiplexer capable of exhibiting flat spectral response characteristics while minimizing insertion loss.
2. Description of the Related Art
The operation of an optical wavelength multiplexer/demultiplexer using an arrayed waveguide grating (AWG) structure can be defined using a grating equation describing dispersion characteristics of incident light resulting from a diffraction of the incident light under the condition in which an array of waveguides is regarded as a diffraction grating. Such an optical wavelength multiplexer/demultiplexer is referred to as an AWG optical wavelength demultiplexer.
Such an AWG optical wavelength demultiplexer is an optical device used in a WDM system to couple optical signals of different wavelengths or to divide an optical signal into those of different wavelengths. Light incident to such an AWG optical wavelength demultiplexer varies in phase while passing through three parts of the AWG optical wavelength demultiplexer, that is, a first slab waveguide, an AWG, and a second slab waveguide. The phase variations of light respectively generated by the parts of the AWG optical wavelength demultiplexer are summed at the final output plane of the AWG optical wavelength demultiplexer, so that a reinforced interference effect is obtained at the final output plane. The above mentioned grating equation is an equation for deriving a condition in which a reinforced interference effect is obtained at the final output plane of the AWG optical wavelength demultiplexer by virtue of the sum of the phase variations. Here, the final output plane is an interface of the second slab waveguide with an output waveguide. Assuming that light is incident to a central input waveguide, the above mentioned grating equation is expressed as follows: EQU n.sub.s d sin .theta.+n.sub.c.DELTA.L=ml [Expression 1]
where, "n.sub.s " represents effective refractive index of the first and second slab waveguides, "n.sub.c " an effective refractive index of the channel waveguides AWG, "d" the pitch of the AWG, "m" the order of diffraction, ".DELTA.L" a length difference between adjacent waveguides in the AWG, and ".lambda." the wavelength of incident light, respectively.
The central operating wavelength .lambda..sub.0 corresponds to the wavelength of light when ".theta." in Expression 1 corresponds to zero. This central operating frequency .lambda..sub.0 is defined as follows: ##EQU1##
From Expression 1, it is possible to derive an equation of a variation in the diffraction angle of light depending on a variation in wavelength. After differentiating both sides of Expression 1 with regard to the wavelength 1, the following Expression 3 is derived: ##EQU2##
Referring to Expression 3, it can be found that a variation in the wavelength of incident light results in a variation in the wavefront direction of the light. Such a variation in the wavefront direction of the light results in a variation in the main lobe position of an interference pattern formed on the image plane of the second slab waveguide.
The spectral response of the optical wavelength demultiplexer can be derived using an overlap integration between the interference pattern formed on the image plane of the second slab waveguide and the mode of the output waveguide connected to the second slab waveguide
However, typical optical wavelength demultiplexers exhibit Gaussian spectral responses because their interference patterns and output waveguide modes have a Gaussian form. When optical wavelength demultiplexers exhibiting such a Gaussian spectral response are applied to a system, it is necessary to accurately control a spectral variation occurring in a laser diode serving as a source for the system. Where such optical wavelength demultiplexers are coupled together in series, a reduction in the passband width of the spectral response occurs between adjacent ones of the optical wavelength demultiplexers. This results in a disadvantage in that the installation and maintenance costs of the system to increase.
In order to solve the above-mentioned problem, the spectral response in each channel should be flat Two methods have been proposed to obtain flat spectral response. The following description will be made in conjunction with these methods.
One method is to adjust the optical path length of the AWG. This method is disclosed in U.S. Pat. No. 5,467,418 issued to Corrado Dragone, Lucent Technologies and is entitled "FREQUENCY ROUTING DEVICE HAVING A SPATIALLY FILTERED OPTICAL GRATING FOR PROVIDING AN INCREASED PASSBAND WIDTH". In accordance with this method, the field distribution of light incident to the second slab waveguide has the form of a sinc function. A diffraction phenomenon occurring in the second slab waveguide can be regarded as a Fourier transform of incident light occurring at the output plane. In order to obtain a flat output profile, accordingly, the above method is adapted to adjust the profile of incident light to have the form of a sinc function corresponding to an inverse Fourier transform of a desired output. In order to obtain such an incident light profile in this method, it is necessary to adjust the lengths of waveguides in the AWG in such a fashion that there is a length difference corresponding to a half-wavelength in at least a portion of the AWG region while intentionally giving loss in accordance with the envelope thereof. For this reason, there is a disadvantage in that the entire device involves additional insertion loss corresponding to the intentional loss given to the AWG.
Another method is to apply a parabolic horn waveguide to an input waveguide coupled to the first slab waveguide of a wavelength demultiplexer in order to obtain flat spectral response characteristics. This method is disclosed in a patent application filed by K. Okamoto, NTT, Japan. The method proposed by K. Okamoto, et al. is disclosed in detail in an article "FLAT SPECTRAL RESPONSE ARRAYED WAVEGUIDE GRATING MULTIPLEXER WITH PARABOLIC WAVEGUIDE HORNS", Electronics Letters, 32, pp. 1961-1962, 1906.
In accordance with this method, the parabolic horn waveguide utilizes the characteristics of the wavelength demultiplexer allowing the mode profile at the first slab input plane to be reconstructed at the output image plane of the second slab waveguide, as it is, thereby forming the input waveguide mode profile into a double peak profile while obtaining a flat final spectral response at the output plane using an overlap integration for the double peak profile. Although it is unnecessary to give an intentional loss, as in the afore mentioned method, this method inevitably involves loss resulting from the fact that the double peak image at the output image plane does not correspond to the local mode of the output waveguide.
As apparent from the above description, both the above mentioned conventional methods inevitably involve additional loss of 2 to 3 dB, as compared to the case involving a Gaussian spectral response, because they are adapted to only vary the image at the image plane while still maintaining the mode of the output waveguide.